A process for producing groundwood pulp by grinding logs with a stone grinder is well known. However, this procwss is disadvantageous in the following points.
1. It is difficult to obtain logs in an amount which is sufficient for industrial use.
2. It is relatively difficult to control the quality of the resultant product.
3. A relatively large amount of labor is required to carry out the process.
Recently, the timber resources of the world are being steadily depleted and, therefore, every country which exports logs tends to restrict the amount of logs exported. Accordingly, there is little question that obtaining logs as raw material for the pulp and paper industry will become increasingly difficult in the future.
Under these circumstances, in the last few decades, a process for producing groundwood pulp from wood chips by using a refiner has been developed and become popular in the pulp and paper industry. This process is referred to as a refiner process and the resultant product is referred to as a refiner groundwood pulp, hereinafter.
In addition, a process for thermomechanically producing groundwood pulp has been developed and is becoming popular in the pulp and paper industry. This process is an improvement of the refiner process and is referred to hereinafter as a thermomechanical process, and the resultant product is referred to hereinafter as a thermomechanical pulp.
The specific power consumption necessary for producing one ton (air-dry weight) of the each of the stone groundwood pulp from logs, the refiner groundwood pulp and the thermomechanical pulp, and the properties of each of these pulps, are shown in Table 1.
TABLE 1 ______________________________________ Pulp Stone ground- Refiner Thermo- wood ground- mech- pulp wood anical Item from logs pulp pulp ______________________________________ Specific power consumption 1100 1800 2000 KWH/Ton A.D. Product Freeness mlCSF 80 100 100 Percentage of fiber fraction Remaining on 24 mesh screen 15 25 37 Passed through 150 mesh screen 52 28 35 Bulk density (g/cm.sup.3) 0.41 0.35 0.38 Breaking length (km) 2.6 3.2 3.8 Tear factor 45 60 85 Scattering coefficient 800 700 620 ______________________________________
Table 1 clearly shows that the refiner groundwood pulp and the thermomechanical pulp are advantageous in having a high freeness, a large breaking length which is due to the large content of the long fibers remaining on the 24 mesh screen, and a high tear factor. However, they have the following disadvantages.
1. The specific power consumption of the refiner groundwood pulp or the thermomechanical pulp is very large, that is, about twice that of the stone groundwood pulp from logs having the same freeness as that of the refiner groundwood pulp or the thermomechanical pulp.
2. The resultant pulp from the refiner process or the thermomechanical process has a high content of the long fibers remaining on the 24 mesh screen, and a low content of the small sized fibers passed through the 150 mesh screen. Accordingly, when this type of the pulp is used for producing paper, the surface of the resultant paper has a relatively poor smoothness.
3. Due to the property of the pulp mentioned in the item 2, above, the resultant paper has a relatively low degree of opacity.
Usually, paper for newspapers is produced from a pulp composition consisting of two or more different types of pulps, including a mechanical pulp such as the refiner groundwood pulp or the thermomechanical pulp. If the pulp composition for producing the paper for newspapers contains, as the mechanical pulp component, refiner groundwood pulp or thermomechanical pulp alone, which contains relatively large amounts of long fibers and a relatively small amount of small sized fibers, the resultant paper for newspapers will have a poor opacity and a low degree of smoothness. Also, since the production of the refiner groundwood pulp or the thermomechanical pulp needs a large specific power consumption, the use of the refiner groundwood pulp or the thermomechanical pulp results in a large comsumption of energy as a whole being required for the production of paper for newspapers.
In order to offset the drawbacks of the refiner groundwood pulp and the thermomechanical pulp, it is preferable that stone groundwood pulp from logs be used together with the above-mentioned pulps. However, as mentioned hereinbefore, it is difficult to procure sufficient logs for the stone groundwood pulp from logs. Accordingly, in order to obviate all of the above-mentioned problems, it is desirable to provide a new process for producing, from wood chips, a groundwood pulp having the same quality as the stone groundwood pulp from logs at the same specific power consumption as that of the process for stone grinding the logs.
Previously, the production of the stone groundwood pulp from wood chips has had the following problems.
1. It is well-known that wood material is composed of a large number of fibers which are orientated substantially parallel to each other along the longitudinal axis of the wood material. Accordingly, in the case where a wood material is ground by a stone grinder, the contact of the wood material with the peripheral surface of the cylindrical grinding stone may be effected in any of the following three manners.
A. The direction of the orientation of the fibers in the wood material is substantially parallel to a shaft around which the stone grinder rotates. In this case, the grinding process can be carried out with an excellent efficiency and the resultant groundwood pulp has an excellent quality.
B. The direction of the orientation of the fibers in the wood material is substantially parallel to a line tangent to the peripheral surface of the grinding stone. In this case, the resultant groundwood pulp has an excellent quality, but the productivity of the grinding process is relatively poor.
C. The direction of the orientation of the fibers in the wood material is substantially parallel to the radius of the cylindrical stone. In this case, the productivity of the grinding process is good, but the quality of the resultant groundwood pulp is poor because the fibers in the wood material are broken during the grinding operation.
When logs are used as the wood material, the grinding is usually carried out in the manner described in item A, above. However, when a group of wood chips are fed to the stone grinder, the wood chips will contact the peripheral surface of the grinding stone in random, various manners, including the above-mentioned manners A, B, C and various intermediate manners between the manners A, B, C. Accordingly, it is natural that the quality of stone groundwood pulp produced from wood chips is poorer than that of stone groundwood pulp from logs.
2. In the case where the logs are ground by a stone grinder, the discharge of the resultant pulp from the grinding region while maintaining the log in the grinding region can be easily attained by providing guide plates located close to the grinding surface of the stone to form peripheral partitions surrounding the grinding region. However, in the case where small sized wood chips are fed to the stone grinder, portions of unground wood chips and incompletely ground wood chips can easily pass through the clearance between the peripheral surface of the grinding stone and the lower ends of the guide plates during the grinding operation. Naturally, in such a case, it is necessary to prevent the undesirable discharge of the unground and incompletely ground wood chips from the grinding region.
Accordingly, in order to utilize wood chips for producing stone ground wood pulp having the same quality as that of the stone groundwood pulp from logs at the same specific power consumption as that of the log grinding process, the above-mentioned problems must be obviated.